This project will lead to the development of an optimized X-ray tube with 19 focal spots that can be placed immediately below and around the multi-leaf collimator (MLC) of a radiotherapy system. By steering a focused electron beam (e-beam) on a series of 19 cooled tungsten targets in sequence, this system will provide a set of projection images that can be reconstructed into a 20 cm 20 cm 20 cm digital tomosynthesis (DTS) volume image that surrounds the radiation target. The projection X-rays are captured by a universal kV-MV imager that can record portal MV images interlaced with the sequence of tomographic kV images. Since the sources are positioned around the perimeter of the MLC, the resulting tomographic cross section images are aligned perpendicular to the axis of the therapy beam. This geometry is referred to as Beam's Eye View imaging. The DTS images will be used to provide precise reference coordinates of a tumor surface for programming of the MLC, in near real-time. The preliminary specifications for the system indicate that a repetition rate of1 volume image per second with near 1 mm spatial resolution will be possible. The multi-source e-beam X-ray tube will be developed by substantially the same premier team who developed the original Imatron EBCT scanners and the more recent e-beam luggage scanner. The e-beam team consisting of TeleSecurity Sciences, Inc. scientists, consultants, and vendors has partnered with the premier cone beam CT team at UC Davis along with a key Varian consultant, who are authorities on flat panel X-ray imagers, and in particular the kV-MV imager that will be used in this project to provide simultaneous MV portal imaging with interlaced kV DTS imaging of the tumor position. Phase 1 of the project will extend the preliminary work to more precisely define the final design specifications for a future commercial system that will be appropriate for the entire radiotherapy industry. Phase 2 funding will develop a suitable prototype of the multi-source e- beam X-ray tube which will demonstrate the feasibility and performance of the commercial system. The majority of radiotherapy vendors have provided guidance and letters of support for this project. The commercial implementation of the proposed system in future radiotherapy systems will permit treatment plans that will have far improved localization of radiation exposure within reduced margins beyond the apparent boundaries of a malignant mass with a corresponding reduction in exposure to surrounding normal structures. This will be especially important in treating tumors of the chest and upper abdomen during free breathing where periodic motion in the range of 2 cm to 5 cm is often encountered. [1]